Approaches
Incremental Modernization
Replacing an old system one capability at a time, with old and new running side by side until the old one has nothing left to do. Each piece is in use as soon as it moves.
Part of Maintenance & Modernization
Incremental modernisation moves a legacy system's capabilities to new code one at a time. Requests for one function are routed to the new code while the rest keeps running on the original system, until the old system can be switched off.
Compared with a full rewrite
A full rewrite has to reproduce years of accumulated behaviour before anything can switch over. Until then the business gets nothing new, the old system still needs maintaining, and the two drift apart as the original keeps changing. With an incremental approach, each piece is in use as soon as it moves, and the order of work can change if priorities do.
What it needs
It needs a point where requests can be routed, such as an API gateway, a proxy or a facade over the old system. It also needs a clear rule for which system owns each set of data during the transition, since errors appear when both systems write the same records. Parts that are stable and cheap to run may stay on the old system for longer.
Running two systems costs more for a while
Until the last function moves, the team runs, monitors and supports two systems, which costs more than running one. We plan the order of moves at the start, including a date for retiring the old system.